1. Field of the Invention
This invention relates to the fabrication of hard disk drives (HDD), particularly to a method of controlling slider fly height by improving control of the air-bearing surface (ABS) shape profile during HDD fabrication.
2. Description of the Related Art
With the introduction of the hard disk drive (HDD) in a wide range of consumer applications, there has been a constant shrinkage in its form factor. This shrinkage has ultimately affected all components of the drive. In particular, the read/write head and its slider assembly has also shrunk several orders of magnitude, which has led to the appearance of certain reliability issues that seriously affect the slider/drive manufacturer. Along with the shrinkage in the drives, there has also been an increased capability for the storage of information.
Traditionally, the direction taken in trying to achieve this high density information storage has been to decrease the magnetic spacing between the disk and the slider. FIG. 1 is a schematic illustration showing a suspension-mounted slider (collectively termed a “head gimbals assembly (HGA)”) positioned above a rotating magnetic hard disk during disk-drive operation at ambient operating temperature. The suspension (10) holds the slider (20) at an angle above the surface of the spindle-mounted (30) magnetic disk (40), producing a magnetic spacing (50) between the edge of the slider and the disk. It is noted that FIG. 1 is quite similar to FIG. 1 of Satoh et al. (U.S. Patent Application Publication No. US 2003/0053256) which teaches a slider that is bonded to a suspension by means of a flexure having a resiliently deformable tongue, the purpose being to improve electrical conduction between the head and the associated electrical assemblies and to prevent electrostatic discharge problems.
The present level of information storage on the disk surface necessitates a magnetic spacing on the order of nanometers, which introduces challenges to the manufacturer in terms of maintaining very tight control over slider fly height (the height of the slider above the disk during disk rotation) as well as over the shape profile of the slider surface (the surface of the slider adjacent to the disk surface). Both fly height and shape are parameters that are sensitive to the back-end manufacturing process and they must be very tightly controlled to insure efficient performance of the HDD.
One of the important reliability factors in HDD performance is the ability of the HDD to perform well under low temperature conditions, temperatures that are lower than approximately 10° C. Under such conditions, the slider profile changes significantly (eg. the crown of the slider surface acquires a sharper curvature) due to thermal stresses, causing the head within the slider to fly away from the disk surface and reducing the ability of the head to write onto the disk. This phenomenon is termed “cold over write (COW).” FIG. 2 is essentially the same as FIG. 1, except that the situation of HDD operation is not the normal ambient temperature of FIG. 1, but is a low temperature condition. The increased magnetic spacing (50) reflects the effect of the more sharply curved crown as the slider flies above the rotating disk.
A potentially effective method to reduce or eliminate this effect is to control the change of the slider profile over all operating temperatures. It is to be noted that data writing at higher temperatures (approximately 50° C.) does not pose a significant problem, as the slider profile does not change as much at these higher temperatures.
A possible explanation for the change of slider profile is the difference in the coefficient of thermal expansion (CTE) of the slider (CTE=7.5 ppm) and suspension (CTE=17.5 ppm) within the head gimbals assembly (HGA). These coefficient differences could give rise to stresses that develop in the HGA during temperature variations and are transferred to the slider body. Once these stresses appear in the slider, the profile of the slider's ABS will be changed, as indicated in FIG. 3.
FIG. 3 is a schematic illustration of the stresses (arrows) formed at low temperatures within a slider (20), mounted on a suspension (10) by a visco-elastic adhesive (25) and contacted by an electrical connection (70). The compressive stresses on the slider (inward directed arrows), resulting from differences in the coefficient of thermal expansion of the slider material and the suspension material, while the slider is fastened to the suspension by adhesive (15) and contacted by an electrical connection (70), cause the crown (curved region of maximum height) of the ABS surface (60) to curve more sharply than it does at normal operating temperatures (shown as dashed curve (65)). The increased curvature, in turn, increases the magnetic spacing between the slider and the disk and produces the COW condition.
Reducing the sensitivity of the slider profile to temperature-induced changes can be done at the wafer level (before individual sliders are formed). At this level, the slider can be re-designed and wafer materials could be developed that are less sensitive to temperature variations. This would be an expensive solution to the problem. Another solution, possibly equally expensive and time consuming, would be to re-design the suspension and adhesive materials to better accommodate thermal stresses. While these approaches are feasible, they require extensive time and monetary costs.
The need to produce and control slider ABS surface curvature is recognized in the prior art. Tam et al. (U.S. Pat. No. 6,831,249 B2) teaches a method and apparatus for producing very high crown and camber curvature in sliders by application of a pulsed laser beam to the flexed surface of the slider in order to produce known stress changes in the surface. The method is designed to produce such stress flexure without the accompanying stress cracks that are associated with similar methods.
Khlif (U.S. Pat. No. 6,627,909 B2) also teaches a method for adjusting the curvature of a slider ABS by means of light-beam induced stresses. The method includes an apparatus for controlling the amount of curvature by scanning the surface with the same light beam that is used to induce the surface stresses.
As will be disclosed below, the method of the present invention achieves the desired stability of the ABS profile by means of a simple modification of the slider fabrication process presently in use by the inventors.